The present invention relates to a fluid filter assembly and to a method of removing impurities entrained within a fluid which solidify to block the filter at low temperatures. In particular the present invention relates to the use of such a fluid filter assembly in the fuel system of a gas turbine engine.
The fuel system of a gas turbine engine functions to provide the engine with fuel in a form and quantity required to suit all engine operating conditions. A fuel pump delivers fuel to spray nozzles which inject the fuel into a combustion chamber in the form of an atomised spray. The fuel is supplied to the spray nozzles via a fuel filter which serves to remove particulate impurities from the fuel. By keeping the fuel free from impurities the possibility of a blockage occurring within the fuel system is reduced.
A heat exchanger is located adjacent the inlet to the fuel filter. The heat exchanger acts to transfer heat from a flow of oil to the fuel thereby cooling the oil. By transferring heat to the fuel liquid impurities entrained within the fuel, such as water, are heated and pass out of the combustion system.
In many gas turbine engines a proportion of the heated fuel is diverted from the engine fuel system and returned to the fuel tanks. The fuel tanks are located in the aircraft wings and the diverted fuel heats the fuel in the tanks to reduce the potential for ice formation on the wing surfaces. When the fuel is diverted to the tanks the additional fuel flow through the engine fuel system reduces the efficiency of the heat exchanger. If the engine fuel temperature is below a critical value the reduction in the temperature rise in the heat exchanger is such that the outlet temperature is below 0xc2x0 C. and the fuel filter can block with ice. A consequence of this is that eventually the filter becomes totally blocked with ice and a bypass provided around the filter opens exposing the engine to iced fuel. This may lead to eventual malfunction of the engine.
The present invention seeks to provide a fluid filter assembly and method of operating the fluid filter assembly to prevent exposure of the engine to iced fuel.
According to the present invention a fluid filter assembly comprises a fluid tank, a pump, at least one heater and a filter in series fluid flow relationship, means for returning a proportion of the filtered fluid to the fluid tank being provided and incorporating means to control the amount of fluid being returned to the tank, the control means operating to prevent the fluid being returned to the tank when a pressure differential is detected across the fluid filter and the temperature of the fluid is below a predetermined value.
In the preferred embodiment of the present invention the control means operates to prevent the fluid being returned to the tank when a pressure differential of the order of 5 psi is detected across the fluid filter and the temperature of the fluid is below 0xc2x0 C.
Preferably the control means is a fluid metering unit which may be controlled electronically.
In the preferred embodiment of the present invention a differential pressure switch is attached to the fluid filter to detect the pressure differential across the filter and a temperature sensor detects the temperature of the filtered fluid. The temperature sensor may be a thermocouple.
Two heaters may be provided and the heaters are preferably heat exchangers in which heat is exchanged between two fluids which in operation pass therethrough.
In a further embodiment of the present invention a second pump is located at the outlet to the filter which delivers the fluid at a higher pressure to spray nozzles which inject the fluid into a chamber as an atomised spray.
The fluid filter assembly in accordance with the present invention is suitable for use in the fuel system of a gas turbine engine.
A method of melting solidified impurities blocking a fluid filter in a fluid filter assembly, which comprises a fluid tank, a pump, at least one heater and a filter in series fluid flow relationship, means being provided for returning a proportion of the filtered fluid to the fluid tank and which incorporates means to control the amount of fluid passing therethrough, the method comprising the steps of, operating the control means to allow a proportion of the fluid to be returned to the fluid tank, detecting the pressure differential across the fluid filter and the temperature of the fluid, operating the control means when the pressure differential and the temperature reach a predetermined valve to prevent the fluid returning to the tank, preventing the fluid being returned to the tank for a predetermined period of time to reduce the flow of fluid passing through the heater, heating the reduced flow of fluid passing through the heater to a temperature sufficient to melt any solidified impurities in the fluid filter.
In the preferred embodiment of the present invention the control means operates to prevent the fluid being returned to the tank when a pressure differential of the order of 5 psi is detected across the fluid filter and the temperature of the fluid is below 0xc2x0 C. The control means operates to prevent the fluid being returned to the tank for a time period of the order of 3 minutes.